1. Field of the Invention
The present invention relates to a method and to an apparatus for individually monitoring transmission sections of a communications transmission link for the transmission of digital useful signals, in which auxiliary signals formed according to a first formation rule are respectively derived from the useful signals at the start of the transmission sections and are inserted into the digital useful signals, in which comparison signals formed according to the first formation rule are derived from the received useful signals at the respective ends of the transmission sections and are compared to the received auxiliary signals, and error messages are formed with the assistance of monitoring circuits in a prescribed dependency on the result of the comparison.
2. Description of the Prior Art
A method of the type generally set forth above is known from the CCITT recommendations G707-G709 or from the German periodical NTZ, Vol. 41, No. 10, 1988, pp. 570-574. In the known method, byte-structured PCM signals are transmitted together with parity bytes. The parity bytes serve the purpose of identifying the bit error rate during the transmission. With respect to control, the parity from the bits of an entire frame or the sub-frame is formed and is then transmitted in the following frame or sub-frame. The parity byte is monitored and repeatedly formed by every intermediate regenerator. Errors in a transmission link can therefore be limited to the affected regenerator section. A further parity byte, of contrast, serves the purpose of bit error acquisition of a transmission link between two network nodes.
In the one parity byte is respectively newly formed at the beginning of the regenerator section and the further parity byte is respectively newly formed at the beginning of the transmission link, the regenerator sections or, respectively, transmission links can be individually monitored, so that errors in a transmission link can be limited to the affected regenerator section or, respectively, errors on a transmission path can be limited to the affected transmission link.
Further, it is known from the German periodical Telcom Report, Special, Multiplexing & Line Transmission, Vol. 10, Dec. 1987, pp. 107-112, to provide digital transmission systems with a means for inoperation monitoring. The intermediate regenerators each thereby respectively contain a monitoring circuit that monitors infringements of the code rule via the on-going digital sum. Apart from error bursts, conclusions can thus be drawn concerning the bit error frequency at the appertaining location. What is referred to as the error quota is transmitted to the locating means in coded form. The bit error frequency at each individual regenerator is therefore acquired in the locating means. Conclusions concerning the transmission quality of the individual regenerator fields can be drawn in this fashion in a monitoring terminal on the basis of a comparison of the measured bit error frequencies, so that an individual monitoring of transmission selections is also possible.
When a bit error rate of zero is measured in the monitoring circuits given an error-free transmission of the digital signals, then the question arises, however, whether this measured result is based on an error-free transmission of the digital signals or on a faulty operation of the monitoring means. It can therefore prove expedient to undertake a checking of the monitoring means at least given the report of the bit error rate of zero.
The monitoring circuits can be checked in line equipment whose regenerators do not carry out any recalculation of the parity bytes or, respectively, any error correction in that defined code errors are arbitrarily mixed in at the beginning of the transmission link, these not falsifying the useful signals or, respectively, useful bytes. In this case, the bit errors resulting from the defined code errors are recognized and displayed in every regenerator as well as in the line terminal equipment of the receiving side, this enabling the checking of the monitoring equipment. However, a comparison of bit error rates is required for individual monitoring of transmission sections.
In a monitoring device of the type initially set forth wherein auxiliary signals are respectively derived from the useful signals at the beginning of the transmission sections and are inserted into the digital useful signals, an individual monitoring of transmission sections is possible, in fact, by direct bit error measurement. However, the desired checking of the monitoring circuits cannot be undertaken by simply mixing in defined code errors.